1. Field of the Invention
The present invention relates to a spectral characteristic measurement apparatus and a spectral characteristic measurement method capable of lessening influence by stray light.
2. Description of the Background Art
Conventionally, spectrometry has widely been used as a technique for evaluating an illuminant and the like. In a spectral characteristic measurement apparatus used in such spectrometry, a spectrometer (typically, a diffraction grating) generally splits measured light from an illuminant or the like, which is a measurement target, into wavelength components and detects each resultant wavelength component with a detection portion. In order to minimize influence other than light to be measured, the spectrometer and the detection portion are accommodated in a housing.
Actually, however, a result of detection by the detection portion may be affected by irregularly reflected light in the housing, light reflected in a diffused manner at a surface of the spectrometer, light having an order other than a measurement order, and the like. In general, such light is referred to as “stray light”. Various methods have been proposed in order to suppress influence by such unintended stray light.
For example, Japanese Patent Laying-Open No. 2010-117343 discloses an optical characteristic measurement apparatus for measuring a spectrum in a shorter period of time with higher accuracy by calculating a correction value based on signal intensity detected in a correction area (an area where light split by a spectrometer is not incident) and calculating a corrected measurement spectrum by subtracting the calculated correction value from each component value included in a measurement spectrum detected in a detection area (an area corresponding to a surface on which light from the spectrometer is incident).
Japanese Patent Laying-Open No. 11-030552 discloses a method of correcting stray light by accurately estimating influence by the stray light appearing in measurement of light guided from a dispersion optical system of a spectrophotometer with a light receiver having a large number of light-receiving elements as a measurement constant of the spectrophotometer and by eliminating that influence.
Japanese Patent Laying-Open No. 2009-222690 discloses an inexpensive spectral measurement instrument capable of removing stray light from measurement data.
The optical characteristic measurement apparatus disclosed in Japanese Patent Laying-Open No. 2010-117343 is premised on the fact that stray light components are even over the entire wavelength range to which it has detection sensitivity. Depending on an apparatus configuration, however, the stray light components are not necessarily even over the entire wavelength range.
According to the stray light correction method disclosed in Japanese Patent Laying-Open No. 11-030552, it is necessary to calculate a ratio between intensity of a light reception signal measured by each light-receiving element and intensity of a light reception signal measured by the light-receiving element corresponding to the split wavelength, as many times as the number of light-receiving elements constituting a detection portion. Therefore, this method is relatively time-consuming.
Though the spectral measurement instrument disclosed in Japanese Patent Laying-Open No. 2009-222690 is preferred from a point of view of correction of a stray light component with the use of spectral irradiance for Δλ, two filters or a filter group should be used, measurement cannot quickly be conducted, and an apparatus configuration is complicated. In addition, it is difficult to achieve complete match between characteristics of the two filters, and it is difficult to enhance measurement accuracy of split spectra. Moreover, since stray light can be corrected only within a range of cut-off characteristics of the filter, only a part of a wavelength range which can be detected by a detection portion can be used for actual detection.